If you're running Windows 98 and 95 your biggest concern should be drivers.

You're more likely to run into driver problems with a USB device than with a physical card, but either way you're very likely to run into problems (Windows 95 didn't support USB natively, so you would need to make sure you have the update that adds it). Most modern devices are not released with drivers for operating systems that old, nor are they tested with them.

Also on a computer that old you're probably using USB 1.1. On a LAN that will definitely be a bottleneck, for an internet connection it may or may not be depending on how fast your ISP is.

Your best bet is to look for a very old card and check online before buying it to make sure you can find and download the drivers you need. Just because a device says it supports a particular operating system doesn't mean you can actually find the drivers you need.